Synthesis, characterization, and crystal structures of N,N′-bis(2-dialkylaminophenyl)thioureas

Two aryl-substituted thiourea compounds, N,N′-bis[2-(dimethylamino)phenyl]thiourea (1) and N,N′-bis[2-(diethylamino)phenyl]thiourea (2), both exhibit intramolecular hydrogen bonds, corresponding to the N—H resonance acquired from 1H NMR spectroscopy. The other N—H bonds form close contacts with the sulfur atom in an adjacent molecule. Different basicity of the NR 2 substituents accounts for the red shift of the N—H stretch in 1 compared to that of 2 acquired from IR spectroscopy.

Aryl-substituted thiourea compounds with amine groups in the ortho positions are expected to have versatile applications due to the unique hydrogen-bonding interactions, but so far, no such compounds have been reported. Diaryl thioureas with dimethylamine functional groups in the meta or para positions of the aryl substituents have been reported, but their crystal structures are unknown.
This report describes the preparation and crystal structures of N,N 0 -bis(2-dimethylaminophenyl)thiourea (1) and N,N 0bis(2-diethylaminophenyl)thiourea (2). Compounds 1 and 2 were prepared by treating 1,1 0 -thiocarbonyldiimidazole and two equivalents of 2-amino-N,N 0 -dialkylaniline in CH 2 Cl 2 . Methyl and NH resonances for 1 were observed at 2.64 and 8.82 ppm in the 1 H NMR spectrum, whereas singlets at 43.99 and 178.66 ppm in the 13 C NMR spectrum match to methyl and C S resonances (Figs. S1 and S2 in the supporting information). Ethyl and NH resonances for 2 were found at 0.89, 2.89, and 9.14 ppm in the 1 H NMR spectrum, while resonances at 12.47, 48.07, and 176.68 ppm in 13 C NMR spectrum correspond to the ethyl and C S groups (Figs. S3 and S4). In the IR spectra, the NH stretches were observed at 3165 and 3226 cm À1 for 1 and 2, respectively (Figs. S5 and S6). High-resolution ESI-MS data confirmed the formation of 1 and 2 with the desired isotopic patterns (Figs. S7 and S8).

Structural commentary
One of the most noticeable features in both 1 and 2 is the intramolecular hydrogen bonding between one of the thiourea NH moieties and the NR 2 group (R = Me and Et) in the ortho position of the aromatic rings (Figs. 1 and 2). The N2-H2 bond distance of 0.896 (15) Å in 1 is slightly shorter (within error ranges) than the N2-H2 bond distance of 0.905 (15) Å in 2, whereas the N3Á Á ÁH2 distance of 1.957 (17) Å for 1 is more elongated than the N3Á Á ÁH2 distance of 1.864 (15) Å for 2. Bond distance analysis suggests that the hydrogen bonding interaction is stronger in 2, due to the increased basicity of amine with longer chains. The increased hydrogen bonding was also observed in the solution, as demonstrated with the deshielded NH resonance of 2 at 9.14 ppm compared to that for 1 at 8.82 ppm. It is worth noting that, contrary to what is expected, there are no hydrogen bonds between N4 and H2 in both 1 and 2 even as the corresponding NÁ Á ÁH distances are 2.707 (12) and 2.641 (14) Å for 1 and 2, respectively.
Slightly asymmetric C1-N1 and C1-N2 bond distances are observed for the trigonal planar thiourea backbones, presumably due to the intramolecular hydrogen-bonding interactions. The C1-S1 bond distance of 1.6879 (11) Å in 1 is between the values for a double and a single bond, while the sum of bond angles around the thiourea carbon (C1) is 360.0 . In the thiourea backbone, the C1-N2 bond [1.3396 (14) Å ] that is involved in intramolecular hydrogen bonding is slightly shorter than the C1-N1 bond [1.3621 (15) Å ] without the intramolecular hydrogen bonding. The other C-N bond distances, such as C1-N1, C3-N3, C8-N2, and C9-N4 range from 1.41 to 1.43 Å . Similar bond distances and angles were observed for 2. The thiourea backbone contains the C1-S1 bond distance of 1.6921 (11) Å , and C1-N2 and C1-N1 bond distances of 1.3415 (14) and 1.3652 (14) Å , respectively, while the sum of the bond angles around C1 is 360.0 . Finally, the C1-N1, C3-N3, C8-N2, and C9-N4 bond distances range from 1.42 to 1.43 Å . Overall, a similar C1-S1 bond distance is observed within a variation of 0.01 Å between 1 and 2, while both structures exhibit a trigonal-planar geometry around the central carbon (C1). Furthermore, the C1-N2 bonds involved in intramolecular hydrogen bonds are 0.02 Å shorter than the C1-N1 bonds in 1 and 2 that do not participate in the hydrogen bonding.

Supramolecular features
Supramolecular features for 1 and 2 were investigated using Hirshfeld surface analysis with CrystalExplorer 21.5 (Spackman et al., 2021). Hirshfeld surfaces for 1 and 2 were mapped over d norm in the range of À0.27 to 1.29 and À0.18 to 1.48 a.u. for 1 and 2, respectively (Figs. 3 and 4). The most intense red spots on the surface indicate the intermolecular H1Á Á ÁS1 interactions (Tables 1 and 2) with the graph-set descriptor R 2 2 (8) (Bernstein et al., 1995). The corresponding intermolecular distances of H1Á Á ÁS1 were measured to be 2.506 (14) and 2.677 (16) Å for 1 and 2, respectively. In addition, the acquired N-H stretch from IR spectra red shifted for 1 (3165 cm À1 ) when compared to that of 2 (3226 cm À1 ). Molecular structure of 2 with displacement ellipsoids at the 50% probability level. Hydrogen atoms attached to carbon were omitted from the figure.

Figure 1
Molecular structure of 1 with displacement ellipsoids at the 50% probability level. Hydrogen atoms attached to carbon were omitted from the figure.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3. Upon scrutiny, no appreciable disorder was observed in either structure. The positions of hydrogen on nitrogen atoms were refined, whereas the other hydrogen atoms were optimized using riding models [C-H = 0.93-0.98 Å ; U iso (H) = 1.2-1.5U eq (C)]. For both structures, data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction:

N,N′-Bis[2-(dimethylamino)phenyl]thiourea (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

N,N′-Bis[2-(diethylamino)phenyl]thiourea (2)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.